A smart roadster exhaust upgrade is one of the most accessible and satisfying modifications you can make to the 698cc three-cylinder turbo. Reduce backpressure, free up exhaust flow, and you improve turbo spool, throttle response and — with the right supporting modifications — peak power. This guide covers every stage of the exhaust system, from the turbine outlet downpipe to the tailpipe, explaining what each component does, what aftermarket options exist, how much power you can realistically expect, and what to watch out for on a car that was built in relatively small numbers and whose parts supply is increasingly enthusiast-driven.
Understanding the Stock Smart Roadster Exhaust System
The factory exhaust on the Smart Roadster 452 was designed around emissions compliance and noise regulations rather than outright performance. Starting at the Garrett 1238S turbocharger, exhaust gases pass through a small-bore downpipe, a ceramic-brick catalytic converter positioned close to the turbo, a mid-section flexi-pipe and finally a single-exit rear silencer. The whole system is notably restrictive, particularly for a turbocharged engine — backpressure upstream of the turbine wheel impairs spool speed and lowers the effective pressure ratio the turbo can achieve.
Because the engine sits mid-rear, the exhaust routing is compact but awkward to access. The catalytic converter sits tight against the turbo, which is thermally efficient for catalyst light-off but means any cat-back work requires working in a cramped engine bay. The factory system weighs surprisingly little given how small it is, but it is manufactured from mild steel and corrosion is common on older examples — particularly around the flexi joint and the silencer hangars. Inspect yours before buying any upgrade parts: a rotten mid-section changes your budget considerably.
Downpipe and Decat: The Biggest Single Gain
The downpipe — the section of pipe connecting the turbine outlet to the catalytic converter — is the most restrictive part of the stock system, and replacing or deleting the catalytic converter delivers the largest measurable reduction in exhaust backpressure. On the 698cc engine, a high-flow sports catalytic converter or a straight decat pipe (a blank section of the same diameter as the downpipe, bypassing the cat entirely) can noticeably improve turbo response and mid-range torque.
Measured back-to-back on a rolling road, a decat downpipe on an otherwise standard 60kW car typically yields 4–7 bhp and a worthwhile improvement in throttle pickup from 2,500 rpm onwards. The improvement is greater on cars running higher boost, because the turbine is working harder and backpressure has a proportionally larger effect. If you are also running an ECU remap — and you should be, for any meaningful power build — the combination of a decat and revised fuelling and ignition maps extracts considerably more than either modification alone. Understanding how boost pressure translates to torque on the 698cc helps explain why reducing exhaust restriction at the turbine exit amplifies the effect of any boost increase rather than simply adding to it.
One important caveat: a decat pipe renders the car non-compliant with emissions regulations for road use in most European countries and the UK. Sports cats — high-cell-count metallic substrates rated at 100 or 200 cpsi — are a legal compromise that retain much of the flow improvement whilst keeping exhaust emissions within testable limits. Quality varies hugely; budget units can crack or fail within a season on a car running elevated boost.
Diameter and Material
The stock downpipe internal diameter is approximately 42 mm. Aftermarket units typically step this up to 45–48 mm, which is sufficient for the flow rates involved up to around 110 bhp. Beyond that, 52 mm is occasionally used but offers diminishing returns without a larger turbine housing. Stainless steel (304 or 316 grade) is the correct choice for longevity; mild steel downpipes will corrode within a few seasons if the car is used in winter or wet conditions.
Cat-Back Systems: Sound, Weight and Modest Gains
A cat-back exhaust system replaces everything downstream of the catalytic converter: the mid-section, flexi-pipe and rear silencer. On the Smart Roadster, the gains in outright power from a cat-back alone (without decat) are modest — typically 2–4 bhp — because the catalytic converter itself is the primary restriction. However, cat-back upgrades offer two genuine benefits: a better exhaust note and a weight reduction if you choose a high-quality stainless steel or titanium unit.
The stock rear silencer is a well-packaged but acoustically dull item. Aftermarket silencers can be had from a handful of specialist suppliers who have built units specifically for the 452. The sound character varies from a mild, sports-car burble to something more aggressive depending on internal packing, pipe diameter and exit aperture. Because the engine is mid-rear, the exhaust note is more present to the driver than on a front-engined car, so it is worth listening to recordings before committing. Drone at motorway cruising speeds is a real issue with very open systems — on a car with a fabric roof and no acoustic insulation, interior noise at 70 mph can become genuinely fatiguing.
Custom-fabricated cat-back systems are also an option if you can find a local exhaust fabricator willing to work with the car. Given the relatively short run of the model, off-the-shelf availability is inconsistent, and having a bespoke system made in mandrel-bent stainless steel is sometimes the most practical route.
Combining Exhaust Work with Other Engine Modifications
A smart roadster exhaust upgrade delivers its best results when it is part of a coherent engine build rather than a standalone modification. The 698cc turbo is a highly interdependent system: improve exhaust flow and you lower the backpressure seen by the turbine, which allows the turbo to spool faster and produce boost more freely — but the stock ECU map will continue to limit boost via the pneumatic wastegate regardless. You need a remap to exploit the freed-up flow.
Similarly, if you are running significantly elevated boost, the stock intercooler becomes a thermal bottleneck before exhaust restriction becomes the limiting factor. Our detailed look at whether an intercooler upgrade is worth fitting explains the charge-temperature threshold at which intercooler efficiency starts costing you power. On cars making over 90 bhp, exhaust and intercooler upgrades are best planned together.
For those building seriously — beyond 100 bhp — a turbo upgrade is the next logical step after exhaust and intercooler work. The Garrett 1238S is a capable unit within its design envelope, but it runs out of efficiency margin as you push it. Exploring the available turbocharger upgrade options will show you when the stock unit becomes the constraint rather than the exhaust system. At that level, larger injectors also become relevant — the stock fuelling system struggles to supply adequate fuel at sustained high boost — and the differences between standard and Brabus injectors are worth understanding before mapping a high-power build.
Variant Considerations: 45kW, 60kW, Brabus
Not all Smart Roadsters are equal, and the exhaust upgrade path differs depending on which variant you own. The 45kW Lite is the least rewarding starting point — it lacks an oil cooler, runs lower boost and its ECU calibration is conservative. Exhaust upgrades on a 45kW car are rarely worth the money unless you are simultaneously remapping and addressing the oil cooling deficiency.
The 60kW standard is the best platform for exhaust work. It has the oil cooler, runs 1.09 bar of boost from the factory, and responds well to decat and remap combinations. The 66kW SB2 Brabus and 74kW full Brabus already run higher factory boost (1.33 and 1.43 bar respectively) and have better breathing hardware from the factory, so the marginal gain from exhaust work is proportionally smaller — though on a Brabus being pushed harder, the decat’s backpressure reduction still pays dividends at the turbine.
One emissions point worth noting: Brabus models were fitted with a slightly different catalyst specification. If you are sourcing a sports cat for a Brabus, confirm fitment compatibility with your supplier before ordering.
Installation Notes and Common Mistakes
Fitting a smart roadster exhaust upgrade is a DIY-achievable job for anyone comfortable working under a car, but the mid-rear engine position and tight packaging create specific challenges. The turbo outlet flange is notorious for seized fasteners — the combination of heat cycling and mild steel hardware means bolts shear easily. Apply penetrating oil liberally 24 hours before attempting removal, use a quality six-point socket (not a twelve-point), and budget time for stud extraction if things go wrong.
Gaskets deserve attention. The turbine outlet gasket sees extreme temperatures and must be replaced at every disassembly. Use a high-temperature exhaust gasket rated for turbo applications rather than a standard manifold gasket. Lambda sensor (oxygen sensor) threads should be cleaned and treated with anti-seize compound on reassembly — the secondary air injection system on the 452 is already a source of fault codes when disturbed, so avoid creating additional issues by cross-threading a sensor on a new downpipe.
After any exhaust work, a short heat-cycling run followed by retorquing of all accessible flanges is good practice. New stainless systems will expand and contract differently from the factory steel items as they bed in. Check for leaks — even a small pre-cat exhaust leak will confuse the upstream lambda sensor and cause fuelling errors.
For those also interested in authoritative technical context, the Bosch Lambda Sensor technical reference explains how oxygen sensors function and why exhaust leaks corrupt their readings. Background on how exhaust backpressure affects turbine efficiency is well-documented in NASA’s turbine engine educational resource, and the engineering principles behind mandrel bending versus crush bending in exhaust fabrication are covered thoroughly by SAE International’s research on exhaust system design.
Realistic Power Expectations
To summarise the numbers honestly: a decat downpipe alone on a 60kW car gains roughly 4–7 bhp. A decat combined with a stage one remap moves the needle to 85–95 bhp depending on map calibration. Add intercooler and further boost work and you are approaching 100–110 bhp. A cat-back without decat adds 2–4 bhp. These are rolling road figures from comparable builds, not manufacturer claims — individual results vary with ambient conditions, fuel quality and the baseline condition of the engine.
What an exhaust upgrade reliably delivers that the dyno chart does not fully capture is improved subjective response: sharper throttle pickup, a more linear power delivery and — if you choose a good silencer — a soundtrack that finally does justice to the car’s sporting character.
Smart Roadster Exhaust Upgrade: Final Recommendation
A smart roadster exhaust upgrade is most valuable when approached as part of a planned modification sequence rather than a standalone bolt-on. Start with a quality stainless decat downpipe or high-flow sports cat, pair it with a professional ECU remap, and you have a meaningfully faster car for a modest outlay. Cat-back work adds character and longevity but should not be your first priority if budget is limited. Always use quality materials, replace gaskets and lambda sensor threads with care, and plan your exhaust upgrade alongside your boost and fuelling strategy for the best results.
